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Organometallics 1997, 16, 4020-4022
CoCF3+ Is Really (FCo+‚‚‚F2C) Quan Chen, Chuan-Yuan Lin, Huiping Chen, and Ben S. Freiser* H. C. Brown Laboratory of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received March 5, 1997X Summary: Gas-phase ion chemistry studies and density functional calculations reveal that the structure of CoCF3+ is really a [FCo+‚‚‚F2C] ion-dipole complex with electrostatic bonding between FCo+ and CF2. The reactions of CoCF3+ with C3-C7 alkanes, alkenes, and other reagents, such as benzene, water, and acetonitrile, yield predominantly CF2 displacement products. This represents the first case of gas-phase C-F activation by bare cobalt ions. Introduction Previous studies have shown that the reactivity of a metal ion in the gas phase can be dramatically altered by the presence of a ligand.1-5 In this paper we report on CF3 as a ligand. CF3, the fluorine-containing analog of methyl radical, is a stable functional group of most hydrofluorocarbons (HFCs), including CF3CF2H, CF3CFH2, and CF3CH3. However, in contrast to CH3, little is known about the effect of CF3 on the reactivity of bare metal ions. Since fluorine has the greatest electronegativity, it exhibits a strong electron-withdrawing effect. On the other hand, fluorine is also a good π-electron donor to carbon π-systems, due to the similar size of its lone-pair 2p orbital to that of carbon. Thus, the effect of fluorine substitution is a combination of inductively withdrawing and π donating.6 In particular, Halle, Armentrout, and Beauchamp have studied the effects of fluorine substitution on carbene stability, as well as the metathesis reactions of fluorinated olefins with NiCH2+ and NiCF2+.5 They determined that D0(Ni+-CF2) ) 47 ( 7 kcal/mol, which is substantially lower than D0(Ni+-CH2) ) 86 ( 6 kcal/mol. On a related topic, C-F activation by metal centers in solution,8-10 on surfaces,11 and in the gas phase12-15 has attracted a great deal of interest due to the very strong C-F bond and the high electronegativity of fluorine. In Abstract published in Advance ACS Abstracts, August 1, 1997. (1) For reviews, see: (a) Eller, K.; Schwarz, H. Chem. Rev. 1991, 91, 1121. (b) Freiser, B. S. Acc. Chem. Res. 1994, 27, 353. (c) Organometallic Ion Chemistry; Freiser, B. S., Ed.; Kluwer Academic Publishers: Dordrecht, 1996. (d) Gas-Phase Inorganic Chemistry; Russell, D. H., Ed.; Plenum Press: New York, 1989. (e) Freiser, B. S. J. Mass Spectrom. 1996, 31, 703. (2) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1984, 106, 3891. (3) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1985, 107, 5876. (4) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1986, 90, 5736. (5) Halle, L. F.; Armentrout, P. B.; Beauchamp, J. L. Organometallics 1983, 2, 1829. (6) (a) Dolbier, W. R. Chem. Rev. 1996, 96, 1557. (b) Brahms, D. L. S.; Dailey, W. P. Chem. Rev. 1996, 96, 1585. (7) Freiser, B. S. Talanta 1985, 32, 697. (8) (a) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. Rev. 1994, 94, 373. (b) Harrison, R. G.; Richmond, T. G. J. Am. Chem. Soc. 1993, 115, 5303. (9) (a) Burdeniuc, J.; Crabtree, R. H. J. Am. Chem. Soc. 1996, 118, 2525. (b) Burdeniuc, J.; Crabtree, R. H. Science 1996, 271, 340. (10) (a) Aizenberg, M.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 8674. (b) Aizenberg, M.; Milstein, D. Science 1994, 265, 359. X
S0276-7333(97)00184-2 CCC: $14.00
this study, we report on the gas-phase reactions of CoCF3+ with small aliphatic alkanes and alkenes, as well as with benzene, water, and acetonitrile. These reactions, together with density functional calculations and the corresponding thermochemistry, indicate that CoCF3+ is really the ion-dipole complex [FCo+‚‚‚F2C]. Results and Discussion While CoCF3+ was first assumed to consist of a trifluoromethyl-Co+ structure 1, it soon became apparent that it might instead be a fluoro-difluorocarbene structure 2. Collision-induced dissociation (CID) and
sustained off-resonance irradiation (SORI) yield difluorocarbene loss as the major fragmentation pathway over the range of kinetic energies studied (5-80 eV in laboratory coordinates for CID and 3-5 eV for SORI), suggestive of a possible fluoro-difluorocarbene structure 2. For comparison, CID and SORI of CoCH3+ yield Co+ 2,3 exclusively by loss of an intact CH3. Even stronger evidence for structure 2 is the reactions of CoCF3+ with L ) alkanes, alkenes, benzene, water, and acetonitrile, which generally result in a prominent CF2 displacement product, CoFL+. These results strongly suggest that CF3 is not an intact ligand in this case and that the CF2 group is bound weakly to Co+, in contrast to the Co+-methyl structure of CoCH3+.2,3 The primary reactions of CoCF3+ with alkanes and alkenes are summarized in Table 1. Like Co+ and CoCH3+, CoCF3+ is unreactive with methane and ethane. Reactions with propane and other linear and branched alkanes up to C7, however, yield the displacement products F-Co+-(CnH2n+2) as the predominant products, except for the reactions with 2-methylpropane and 2-methylbutane. CID of these product ions yields FCo+ exclusively, indicating that the alkane remains intact on the metal center. The displacement reaction implies that D0(FCo+-F2C) e D0(FCo+-C3H8). While the latter value is not known, D0(Co+-C3H8) ) 30.9 ( 1.4 kcal/ (11) (a) Pradeep, T.; Ast, T.; Cooks, R. G.; Feng, B. J. Phys. Chem. 1994, 98, 9301. (b) Pradeep, T.; Riederer, D. E., Jr.; Hoke, S. H., II; Cooks, R. G.; Linford, M. R. J. Am. Chem. Soc. 1994, 116, 8658. (c) Chen, G.; Hoke, S. H., II; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1994, 139, 87. (12) Dietz, T. G.; Chatellier, D. S.; Ridge, D. P. J. Am. Chem. Soc. 1978, 100, 4905. (13) Bjarnason, A.; Taylor, J. W. Organometallics 1989, 8, 2020. (14) Garcia´, E.; Huang, Y.; Freiser, B. S. Inorg. Chem. 1993, 32, 3595. (15) (a) Schro¨der, D.; Hrusa´k, J.; Schwarz, H. Helv. Chim. Acta 1992, 75, 2215. (b) Cornehl, H. H.; Hornung, G.; Schwarz, H. J. Am. Chem. Soc. 1996, 118, 9960. (16) Cody, R. B.; Burnier, R. C.; Reents, W. D., Jr.; Carlin, T. J.; McCrery, D. A.; Lengel, R. K.; Freiser, B. S. Int. J. Mass Spectrom. Ion Processes 1980, 33, 37.
© 1997 American Chemical Society
Notes
Organometallics, Vol. 16, No. 18, 1997 4021
Table 1. Percentage Abundance of Primary Products Observed in the Reactions of CoCF3+ with Various Alkanes and Alkenes alkane or alkene methane ethane propane n-butane 2-methylpropane n-pentane 2-methylbutane 2,2-dimethylpropane n-hexane n-heptane ethene propene 1-butene isobutene cis-2-butene 1,3-butadiene
neutral lost
CF2 CH4 CF2 CoCF3H CH4 CF2 CF2 CoCF3H CF2 CF2 CF2 CF2 CF2 CF2 CF3H CF2 CF3H CF2 CF3H CF2 CF2
products
rel %
no reaction no reaction CoF(C3H8)+ CoCF3(C3H6)+ CoF(C4H10)+ CoCF3(C4H10)+ C4H9+ CoCF3(C3H6)+ CoF(C4H10)+ CoF(C5H12)+ C5H11+ CoF(C5H12)+ CoF(C5H12)+ CoCF3(C5H12)+ CoF(C6H14)+ CoF(C7H16)+ CoF(C2H4)+ CoF(C3H6)+ Co(C3H5)+ CoF(C4H8)+ Co(C4H7)+ CoF(C4H8)+ Co(C4H7)+ CoF(C4H8)+ CoF(C4H6)+
100 75 18 7 70 18 12 100 71 29 71 29 100 100 100 92 8 93 7 94 6 100 100
mol29 provides a useful estimate. Interestingly, the hydride abstraction products i-C4H9+ and i-C5H11+, which are stable tertiary carbocations, are dominant for the reactions of CoCF3+ with 2-methylpropane and 2-methylbutane, respectively. Formation of i-C4H9+ suggests that D0(CoCF3+-H-) > D0(C4H9+-H-) ) 376.1 kcal/mol.30 For comparison, CoCH3+ reacts with alkanes larger than ethane by initial C-H insertion, followed by CH4 and H2 loss to yield Co(allyl)+ species.2 Pseudo-first-order kinetics are observed for the reactions of CoCF3+ with propane, n-butane, n-pentane, n-hexane, and n-heptane. For example, the kinetics plot of CoCF3+ with n-heptane at ∼2.5 × 10-7 Torr is shown in Figure 1. [A] is reactant ion intensity after time t, and [A0] is obtained by summing the intensities of the (17) Collision Spectroscopy; Cooks, R. G., Ed.; Plenum: New York, 1978. (18) Gauthier, J. W.; Trautman, T. R.; Jacobson, D. B. Anal. Chim. Acta 1991, 246, 211. (19) Allison, J.; Ridge, D. P. J. Am. Chem. Soc. 1979, 101, 4998. (20) Carlin, T. J.; Freiser, B. S. Anal. Chem. 1983, 55, 571. (21) Comisarow, M. B.; Grassi, V.; Parisod, G. Chem. Phys. Lett. 1978, 57, 413. (22) Bartmess, J. E.; Georgiadis, R. M. Vacuum 1983, 33, 149. (23) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (24) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976. (25) (a)Becke, A. D. Phys. Rev. 1988, A38, 3098. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (c) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (26) Kais, S.; Herschbach, N. C. H.; Murray, C. W.; Laming, G. J. J. Chem. Phys. 1993, 99, 417. (27) Siegbahn, P. E. M. Adv. Chem. Phys. 1996, 333. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. W.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Anfres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, revision D.1; Gaussian, Inc.: Pittsburgh PA, 1995. (29) Armentrout, P. B.; Kickel, B. L. In Organometallic Ion Chemistry; Freiser, B. S., Ed.; Kluwer Academic Publishers: Dordrecht, 1996; Chapter 1. (30) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Gas-Phase Ion and Neutral Thermochemistry. J. Phys. Chem. Ref. Data, 1988, 17, Suppl. No.1.
Figure 1. Pseudo-first-order plot of the reaction of CoCF3+ with n-heptane at 2.5 × 10-7 Torr. Table 2. Rate Constants and Calculated Reaction Efficiencies for the Reactions of CoCF3+ with Various Linear Alkanesa reagent
kobs
propane n-butane n-pentane n-hexane n-heptane
1.3 × 4.8 × 10-12 5.9 × 10-11 2.0 × 10-10 2.6 × 10-10
a
10-12
k
L
reaction efficiency
1.0 × 10-9 1.1 × 10-9 1.1 × 10-9 1.1 × 10-9 1.2 × 10-9
0.1% 0.5% 5.4% 17% 22%
Rate constants have the units of cm3 molecule-1 s-1.
Table 3. Bond Lengths and Bond Angles for the Optimized Geometries of CoCF3+ Using DFT with the Standard Effective Core Potential for Co+ and the Dunning-Hay Double-ζ Basis Set for C and Fa species CoCF3+
a
bond length (Å) C-F(1) C-F(2) Co-F(2) Co-F(3)
1.291 1.629 1.948 1.726
bond angle (deg) F(1)-C-F(2) C-F(2)-Co F(2)-Co-F(3)
100.2 155.0 179.4
See Figure 2.
reactant ion and product ions at each time. The slopes of the pseudo-first-order plots are used with the calibrated reactant pressure to obtain the observed rate constants, kobs. The Langevin rate constants, kL, are also calculated to determine reaction efficiencies.31 The values obtained for kobs, kL, and the reaction efficiencies are listed in Table 2. The linear pseudo-first-order kinetics observed for each of the reactions suggest, although not unequivocally, that the CoCF3+ ions are thermalized and consist of one isomeric structure. As the polarizability of the alkane increases, the reaction rates and efficiencies between CoCF3+ and the alkane increase dramatically. These results are again consistent with a weakly bound FCo+‚‚‚F2C complex. The reactions of CoCF3+ and alkenes are also dominated by CF2 displacement, in contrast to those of CoCH3+, which proceed by initial elimination of methane to form an activated π-allyl complex.3 CID of the CoFL+ product ions gives FCo+ as the major fragment and loss of HF as the minor fragment. The CF2 displacement reaction of CoCF3+ with ethene suggests that D0(FCo+-F2C) < D0(FCo+-C2H4) ≈ D0(Co+-C2H4) ) 42.9 ( 1.6 kcal/mol.29,32 In addition, we have performed density functional calculations on CoCF3+. Four possible structures 3-6 (31) Su, T.; Bowers, M. T. In Gas Phase Ion Chemistry; Bowers, M. T. Ed.; Academic Press: New York, 1979, Vol. 1, p 83. (32) Haynes, C. L.; Armentrout, P. B. Organometallics 1994, 13, 3480. Note that the presence of fluorine on the cobalt center makes the equality strictly an estimate.
4022 Organometallics, Vol. 16, No. 18, 1997
Figure 2. Optimized geometry of CoCF3+ with Becke-3LYP using the standard effective core potential for Co+ and the Dunning-Hay double-ζ basis set for C and F. All distances are given in Å.
were proposed as the starting structures in this calculation. Only structure 4 is found to be a true minimum,
while the other minimum structures of CoCF3+ were not detected. The ground state of Co+ is found to be 3F. Spin contamination is small in all of the calculations and the deviation of 〈S2〉 is less than 1%. The optimized parameters are presented in Table 3, and the optimized structure is shown in Figure 2. The optimized CoCF3+ exhibits Cs symmetry with a Co-F(3) distance of 1.726 Å and a slightly elongated CF2 unit bound to Co through F(2) by a distance of 1.948 Å. The Mulliken population analysis gives the charge distribution as follows: q ) +1.117 for Co, q ) +0.578 for C, q ) +0.015 for F(1), q ) -0.326 for F(2), q ) -0.383 for F(3). The calculated binding energy of FCo+-F2C is 30.9 kcal/mol, which is in accordance with the experimental limit to a first approximation of 30.9 ( 1.4 kcal/mol. Interestingly, Paulino and Squires have reported a similar finding for CF2Cl-, where the evidence points to a carbene-halide anion structure consisting of a free carbene moiety bound electrostatically through carbon to Cl- with nearly a full -1 charge on Cl atom.33,34 In a similar vein, recent theoretical calculations by Schleyer and coworkers indicate that AH3+ is actually HA+‚‚‚H2 for A ) Sn and Pb, while CH3+ and SiH3+ are strongly bound D3h structures.35 Likewise, our calculations indicate that CoCF3+ is a [FCo+‚‚‚F2C] ion-dipole complex. Conclusions Upon fluorine substitution, the structure and reactivity of CoCF3+ are dramatically different compared to CoCH3+. Both experimental and theoretical results show that the structure of CoCF3+ corresponds to a [FCo+‚‚‚F2C] ion-dipole complex. The weak bonding between FCo+ and CF2 leads to predominant displacement of CF2 by alkanes, alkenes, and other reagents. In contrast, our recent experiments and calculations indicate that CoCF2+ corresponds to a Co+dCF2 structure with D0(Co+-CF2) bracketed as 49 ( 7 kcal/mol (33) Paulino, J. A.; Squires, R. R. J. Am. Chem. Soc. 1991, 113, 1845. (34) Paulino, J. A.; Squires, R. R. J. Am. Chem. Soc. 1991, 113, 5573. (35) Kapp, J.; Schreiner, P. R.; Schleyer, P. V. R. J. Am. Chem. Soc. 1996, 118, 12154.
Notes
and calculated at 50.4 kcal/mol.36 Thus, the bonding, bond strength, and reactivity of these two difluorocarbene complexes are quite different. To the best of our knowledge, this is also the first case of gas-phase C-F activation by bare Co+. Currently, we are examining the structural integrity of CF3 bound to other metal ions, such as the first- and second-row transition metals. Experimental Section All of the experiments were performed with a Nicolet (now Finnigan FT/MS, Madison, WI) prototype FTMS-1000 Fourier transform mass spectrometer, equipped with a Walker Scientific 15-in. electromagnet maintained at 1 T.7 Laser desorption ionization was used to generate Co+ from the pure metal foil by focusing the 1064 nm wavelength of a Quanta-Ray Nd: YAG laser on the metal target.16 Argon was present at a static background pressure of ∼1.0 × 10-5 Torr, serving as a cooling gas to thermalize the ions prior to reactions, as well as the collision gas in collision-induced dissociation (CID)17 and sustained off-resonance irradiation (SORI)18 experiments. In analogy to the synthesis of CoCH3+ from CH3I,19 laserdesorbed Co+ was reacted with trifluoromethyl iodide, which was pulsed into the cell via a General Valve Series 9 solenoid pulsed valve.20 This yielded three products, CoI+, CF2I+, and CoCF3+, reactions 1-3. The desired CoCF3+ was then isolated
by using swept double-resonance ejection techniques21 and cooled 400 ms prior to further reaction. For the kinetics study, the alkane neutrals were introduced into the cell through a Varian leak valve and the reaction time was varied between 200 ms and 2 s. The pressure of the neutral reagent was kept at ∼2.5 × 10-7 Torr, and Ar was used as the cooling gas at a total pressure of ∼1.0 × 10-5 Torr. The pressure of the alkane neutral was measured using standard procedures for calibrating the ion gauge for the sensitivity toward the alkane.22 The uncertainty in the pressure introduces an error of (30% into the measurement of the absolute reaction rate constants, while the relative reaction rate constants are more reliable. The branching ratios of primary product ions are reproducible to within (10%. Theoretical calculations were carried out first at the Hartree-Fock level for full geometry optimization of CoCF3+ using the standard effective core potential23 for Co+, and the Dunning-Hay double-ζ basis set for C and F atoms.24 In order to treat the effect of electron correlation, all of the calculations were repeated using DFT with Becke-3-LYP for the exchange correlation functional.25 This functional has three fitted parameters and includes the Hartree-Fock exchange term. Although this functional is not exact,26 it gives relatively accurate results for bond dissociation energies and geometries of transition metal compounds.27 Corrections for zero point energy have been taken into account, as well as different spin configurations for Co+ including 3F and 5F states. All of the calculations were performed with the Gaussian 94/DFT program package.28
Acknowledgment is made to the Division of Chemical Sciences in the Office of Basic Energy Sciences in the United States Department of Energy (Grant No. DE-FG02-87ER13766). OM970184Z (36) Chen, Q.; Auberry, K. J.; Freiser, B. S. Int. J. Mass Spectrom. Ion Processes, submitted for publication.
